Titanium has a number of applications where its advantageous mechanical properties and its relatively low specific weight are highly appreciated. In some applications it is interesting to use commercially pure titanium instead of the more commonly used alloys such as e.g. Ti-6Al-4V. This is especially interesting in applications where the final product may come in daily contact with human tissue, typically as implants, but also in other forms such as e.g. jewellery, piercings and the like.
This is due to the fact that vanadium, which often is present in Ti-6Al-4V and other mechanically advantageous alloys, is toxic and allergenic and is therefore not suited to be comprised in materials that are to be used as implants or in other similar applications. Further, the biocompatibility of commercially pure titanium is generally recognised as better than that of other titanium alloys.
A problem is however that titanium material with low vanadium content, such as e.g. commercially pure titanium, has markedly lower yield strength and tensile strength than the corresponding alloys.
There is therefore a need for a titanium material with low vanadium content, typically a commercially pure (CP) titanium material, with relatively higher yield and tensile strength than a conventional CP titanium material, and preferably with a conserved high ductility.
It is possible to increase the strength of a CP titanium material by introducing dislocations or by reducing the grain size. However, conventionally, these methods lead to an unwanted reduction of the ductility, which makes the material less suitable for most applications.
Lately, the introduction of nano twins in metal materials has proven to be an effective way to obtain materials with high strength and high ductility. All materials are however not susceptible to such processing. Further, there is no general operation, by means of which nano twins may be induced into a material. Different methods have been shown to have effects on the inducement of nano twins in different materials.
A twin may be defined as two separate crystals that share some of the same crystal lattice. For a nano twin the distance between the separate crystals is less than 1000 nm.
From the non-patent-literature document XP-002639666 it is known to strengthen nanostructured titanium at high strain rates. The titanium material is prepared by equal channel angular pressing plus cold rolling. Hence, the titanium material is an ultrafine-grained titanium material. During the deformation process of the titanium material at high strain rates twinning has been observed in the material.
Document US 2005/0109158 relates to a method of preparing ultrafine-grained titanium or titanium alloy articles. Coarse grained titanium materials are severely mechanically deformed using cryogenic milling into an ultrafine-grained powder. The method results in a material with improved mechanical properties.
There is however no known method of improving the strength of titanium that is not formed from powder, such as e.g. casted titanium.